Reinstall windows 7

Recently my laptop is running very slow. I have scan for virus and used malware and found a few Trojan virus. They have all been removed and there has been no improvement. I have backed up my files on an external HD. I would like to reinstall windows 7 or format my HD so my laptop can run like new. I do not have an install cd so I think it's preinstall onto my laptop.

How can I reinstall windows 7?

Thanks for reading my post

 

What type of scan did you use?

I run AVG, and I use MalwareBytes as well. My browser started acting up yesterday, and though AVG found nothing MalwareBytes picked up 11 objects. These days a multi-pronged defense is advisable. I also run Kaspersky's TDSSKiller in case an infection may be a bootkit hiding in the MBR, aka the boot sector.

 

Preinstalled systems have a recovery system which can restore your computer to its factory condition. It's usually invoked by pressing F11 immediately it boots. But it will destroy any files or applications you have created so it's definitely a last resort and you must backup your files first and ensure you have or know where to get the installation media (and keys) for your software.